1. Field of the Invention
The present invention relates to a high frequency circuit board which includes a microstrip line and a coplanar line. Further, the present invention relates to a high frequency circuit board which enables transmission of signals with little loss and reflection to semiconductor device by carrying out conversion of a transmission mode from the microstrip line to the coplanar line efficiently and with low loss.
2. Description of the Related Art
Conventionally, a high frequency circuit board using a conversion of a transmission mode from a microstrip line to a coplanar line as disclosed in Japanese Patent Laid-open Publication No. S64-5102, No. 2001-94012, and No. 2001-102820.
In the Japanese Patent Laid-open Publication No. S64-5102, the configuration as shown in FIG. 9 and FIG. 10 is disclosed as a conventional example. A representative method of an imaginary grounding is shown in FIG. 9. Conversion of a transmission mode between the microstrip line and the coplanar line is realized by making a connecting point between the coplanar line and the microstrip line the imaginary ground using an open stub with λ/4 (λ: the wavelength of high frequency signal). FIG. 10 shows another example of conversion, in which conversion of a transmission mode between the microstrip line and the coplanar line is realized by connecting a ground line end portion of the coplanar line on the front side of the circuit board and a circuit board ground conductor on the reverse side of the circuit board using a through hole on the side closer to the coplanar line circuit board.
In the case where a semiconductor device is mounted on a circuit board formed with the coplanar line, combination of the methods as shown in FIG. 9 and FIG. 10 is conceivable. Specifically, in a connecting region of the microstrip line, a ground line with one end thereof open is arranged on both sides of the line. Next, the other end of the ground line is connected to the ground line of the semiconductor device with a gold wire. Next, an imaginary ground can be realized by making the distance between the connecting point and the open end of the ground line as λ/4 or 3λ/4. As a result of this, mode conversion is enabled. Similarly, imaginary grounding can also be realized at the connecting point by grounding an open end 72 of the ground line to the ground surface on the reverse side, and making the distance between an end point 70 thereof and a connecting point 71 as above mentioned λ/2, as shown in FIG. 11. Accordingly, conversion for both modes is enabled. As described above, the configuration shown in FIG. 11 can be readily imagined from FIG. 9 and FIG. 10.
In the Japanese Patent Laid-open Publication No. 2001-94012, an example of conversion of a transmission mode from the microstrip line to the coplanar line using a fan-shaped ground pattern of the imaginary ground is indicated. The third embodiment and FIG. 9 in the Japanese Patent Laid-open Publication No. 2001-102820 indicate conversion of a transmission mode between the coplanar line and the microstrip line. In addition, the first embodiment and FIG. 3A to FIG. 3D thereof provide a way of preventing increase in preventing in increase reflection due to impedance fluctuation caused by the gold wire. Specifically, an inductor made of a line with a large width and a capacitor made of a line with a small width are formed at the connecting portion, and these elements and an inductor made of gold wire configure a filter. It is described that this enables matching impedance between the lines to be connected.
The conversion methods of the transmission mode in the Japanese Patent Laid-open Publication No. 2001-94012 and in the third embodiment of Japanese Patent Laid-open Publication No. 2001-102820 as shown in FIG. 9 are to form a slit which makes the width of the ground line smaller, and to realize the imaginary ground at the position of the slit. In addition, ingenuity is used for facilitating favorable impedance matching and conversion by configuring a capacitor such that the slit has a larger width in some regions of a signal line so as to decrease the gap from the ground line. However, since the gap between the signal line of the coplanar line and the ground line is only approx. 20 μm even at the narrowest portion, it is very difficult to create it in the case where an organic substrate is used. On the other hand, if the gap at this portion increases, the transmission mode for the coplanar line will not be dominant. Specifically, in the case where an organic substrate is used to decrease the manufacturing cost, the gap which can be created increases. As a result of this, according to the conversion method as described in the Japanese Patent Laid-open Publication No. 2001-94012 and Japanese Patent Laid-open Publication No. 2001-102820, conversion of a transmission mode cannot be performed efficiently, and it is difficult to connect the wire to the line of the semiconductor while the transmission mode for the coplanar line remains unchanged.
In addition, in the configuration shown in FIG. 11 as above described which is conceivable from the conventional example described in the Japanese Patent Laid-open Publication No. S64-5102, conversion of a transmission mode from the microstrip line to the coplanar line occurs rapidly, and thereby increasing a loss. Furthermore, since the necessary distance from the short-circuit end 70 with respect to the ground surface on the reverse side to the line of the semiconductor device as well as to the connecting point 71 of the wire is λ/2, the line will become longer.